492: Improved ultrasound estimation of birth weight in the macrosomic infant by application of a correction factor to the gestation-adjusted projection (GAP) method

To improve estimated birth weight (EBW) determination in macrosomic infants (estimated fetal weight equal or greater than 4000 grams) by application of a correction factor to the gestation-adjusted projection (GAP) method. A review was performed of 274 singleton pregnancies delivered at term. After ultrasounds were performed between 34.0 and 36.9 weeks gestation, an EBW was calculated for each patient using the GAP method (EBWGAP). Using linear regression, a correction factor was developed that minimized the systematic error in EBWGAP. The model was then tested on a second group of 137 patients. Both groups were then combined and our linear regression was repeated to further refine our model. The GAP method systematically overestimated fetal weights of the heavier infants in our population. The model we derived showed improved accuracy compared to the GAP method. When applied to the entire group of 411 patients, our correction to the GAP method improved specificity for macrosomia from 95.1% to 98.4% (p=.02). Stated differently, the false positive rate was reduced from 4.9% to 1.6%. Application of our model to our study population reduced the number of false positive results for fetal macrosomia.